1. Field of the Invention
This invention relates to hydraulic fluid compositions, particularly hydraulic fluid compositions containing borate esters.
2. Description of the Prior Art
In recent years, hydraulic fluid compositions are required to have high boiling points. Thus, they are required to be of the DOT-4 grade, for instance. As those hydraulic fluids that have high boiling points, there are widely known borate ester type fluids. However, the known borate ester type hydraulic fluids are inferior in behavior to rubber to hydraulic fluids of the polyoxyalkylene type. The borate ester type fluids have a drawback that, in a rubber swelling test, they cause formation of a large amount of precipitate. Accordingly, it is feared that, when an automobile is running, for instance, such rubber additives as fatty acids and zinc oxide be extracted from the rubber cup used in the brake mechanism into the brake fluid, causing turbidity or forming a precipitate. Such fatty acids, zinc oxide and other extracted substances may react with metal components used in the cylinder, piping and so on, to cause formation of an insoluble precipitate, which may lead to clogging of the piping or the like.
For dissolving this problem, it has already been proposed to use borate esters having a semipolar structure (e.g. a reaction product of 2 moles of glycerol and 1 mole of boric acid). However, such borate esters are still unsatisfactory.